1. Field of the Invention
The present invention relates to a furnaces for firing ceramics and methods for firing ceramics.
2. Prior Art
To fire ceramics, generally, a box-shaped electric furnace is used. In particular, as shown in FIG. 10, a plurality of blocks of ceramic forms 102, stacked up in plural vertical or lateral layers in a saggar 101, are charged into the furnace, and fired for 20 to 50 hours. However, in this arrangement, the saggar 101 and ceramic forms 102 may react with each other during the firing process. To prevent this, a separator 103 or powder is used. To prevent a mutual sticking of the ceramic forms, powder may be also sprinkled over the forms.
Further, in such a ceramic firing method, due to the difference in position of the saggar 101, the difference between the central part and the peripheral part of the saggar 101, or the stacking position of the ceramic forms 102 stacked up in vertical or lateral layers, a temperature difference will occur. As a result, the firing atmosphere tends to be uneven. Therefore, individual ceramic forms 102 undergo different thermal hysteresis, by being fired in a different atmosphere, and sometimes, the composition distribution in the ceramic forms may be non-uniform. The fired ceramics will differ in deformation and other characteristics. Besides, since the ceramic forms 102 are overlaid and fired at high temperature, if powder is sprinkled, the ceramics will often stick to each other.
An attempt has been made toward improving the fluctuations of thermal hysteresis, or atmosphere, or non-uniformity of composition distribution due to the firing of ceramic forms 102 by placing them in the saggar 101 described above. In Japanese Laid-open Patent 61-101469, it is proposed to fire ceramic forms while rotating and stirring the forms. Specifically, the ceramic forms 112 are placed in a capsule 111, as shown in FIG. 11. In this method, heat is fed continuously into the furnace core tube, which rotates the tubular capsule. However, the capsule is rotated even when the mechanical strength of ceramic forms is extremely low due to the loss of binder by burning in the midst of firing. As a result, the ceramic forms are likely to be broken by the impact of rotation. The rate of breakage is particularly large when the firing ceramic forms are thin sheets or slender columns. Also, if fired by once calcining the ceramic forms to raise their mechanical strength, the forms are still continuously stirred during the firing process, causing the ceramic forms to collide against each other. As a result, surface wear of the ceramic forms is promoted, problems occur in appearance and in electrode formation, and cuts and defects are likely to take place. Moreover, the powder generated by the abrasion of the ceramic forms reacts with the capsule to stick on the inner wall of the capsule. As a result, the inner wall of the capsule may be roughened, or a chemical reaction may be promoted to break the capsule. To avoid this, the rotating speed of the capsule may be lowered. However, then the stirring is insufficient, and sticking, deformation or other defects may occur, the non-uniformity will increase, or other new problems may occur.
On the other hand, Japanese Laid-open Patent 6-273051 discloses, as shown in FIG. 12, a method of calcining and firing, while stirring ceramic forms 123, by disposing a bar 122 along the axis of rotation and displaced from the axial center, in an internal space of a furnace core tube 121 of a continuous heat treatment furnace. The furnace core tube rotates in the peripheral direction. In this method, the ceramic forms are fired while being rotated continuously throughout the process, and also stirred by the bar. According to this method, the problems pointed out above with respect to the method of Japanese Laid-open Patent 61-101469 are further emphasized. In addition, since the ceramic forms are sent into the furnace core tube in bulk state, it is difficult to equalize the thermal hysteresis of the individual ceramic forms, and the non-uniformity of composition is further encouraged.
Besides, the method disclosed in Japanese Laid-open Patent 61-101469 involves another problem. In particular, in order to prevent the ceramic forms 112 from spilling over or covering the air passage holes 113 in the central part of the capsule 111, ceramic forms 112 can be packed to only about 40% of the apparent volume percentage. By contrast, in the method disclosed in Japanese Laid-open Patent 6-273051, which does not use the capsule, the furnace core tube can be massively packed with ceramic forms. However, as the packing amount increases, the weight of the ceramic forms increases, which in turn causes the ceramic forms to increase their breakage and surface abrasion generated by the continuous rotation of the furnace core tube during the firing process. Also, the filling rate of the ceramic forms in the furnace core tube cannot be raised sufficiently.
The present invention is intended to solve the problems described above, and it is an object herein to present a method of firing ceramics and a furnace for manufacturing ceramics with advanced mass producibility, while minimizing appearance defects and characteristic fluctuations such as sticking, deformation, breakage, and surface abrasion in the ceramics.